1. Field of the Invention
The present invention generally relates to fabrication or modification of insulating layers for electrical cables, and more particularly to a method of altering a portion of a cable's semiconductive layer to render it electrically insulative.
2. Description of the Prior Art
A high percentage of high voltage cable installed today is polymeric-based, with a central metal conductor surrounded by a layer of carbon-loaded polymer (a semiconductive layer), further surrounded by a dielectric layer which is also a polymer, and another, outer layer of semiconductive material, for electric field containment. The semiconductive layer is typically made by loading a polymer, such as low density polyethylene, with a large amount of carbon powder (e.g., 30-50% by volume). This high loading of carbon forms chains of overlapping particles to allow conduction through the polymer matrix. There is a dramatic increase in conductivity within a narrow range (usually within about 5 vol. %) in which these conductive chains are established. The volume percent where this increase in conductivity occurs (the percolation threshold) is very dependent on the size and aspect ratio of the conductive particles.
Present technology in cable splicing and terminations for, e.g., 5 kV-25 kV cables typically involves the manual removal of 3 to 6 inches of length of the outer semiconductive layer from the cable end. This step further isolates the center conductor from the grounded semicon so that current does not flashover and destroy the splice or termination. The removal operation requires considerable craft and training to properly accomplish. Scratches or a slightly lifted edge of the semiconductive layer resulting from cable preparation leave air gaps which will breakdown in the high electric field of the energized cable, forming a reactive plasma which degrades the polymer insulation, leading to cable failure, often violent.
There are two major types of electric cables with differing semiconductive layers, strippable or coextruded. The majority of cable in the United States is strippable to allow quicker removal of the layer for splicing or terminating. Although coextruded semicon has better overall electrical characteristics, it is used less because it requires much more extensive work to properly remove the semicon to make a splice or termination. The removal of the semicon has been identified as the largest factor in splice/termination failures. It would, therefore, be desirable to devise a method of preventing flashover to the semiconductive layer at a splice or termination without requiring tedious removal of a portion of the semiconductive layer, and it would be particularly advantageous if the technique were usable with both strippable and coextruded semiconductive layers.